Thermal Recovery of the Electrochemically Degraded LiCoO2/Li7La3Zr2O12:Al,Ta Interface in an All-Solid-State Lithium Battery

All-solid-state lithium batteries are promising candidates for next-generation energy storage systems. Their performance critically depends on the capacity and cycling stability of the cathodic layer. Cells with a garnet Li7La3Zr2O12 (LLZO) electrolyte can show high areal storage capacity. However, they commonly suffer from performance degradation during cycling. For fully inorganic cells based on LiCoO2 (LCO) as cathode active material and LLZO, the electrochemically induced interface amorphization has been identified as an origin of the performance degradation. This study shows that the amorphized interface can be recrystallized by thermal recovery (annealing) with nearly full restoration of the cell performance. The structural and chemical changes at the LCO/LLZO heterointerface associated with degradation and recovery were analyzed in detail and justified by thermodynamic modeling. Based on this comprehensive understanding, this work demonstrates a facile way to recover more than 80% of the initial storage capacity through a thermal recovery (annealing) step. The thermal recovery can be potentially used for cost-efficient recycling of ceramic all-solid-state batteries.


Thermodynamic modelling by DFT
Atomic structures and spin density distribution were visualized with the VESTA program. 1 Spin-polarized DFT calculations were performed using the Projector Augmented-Wave (PAW) pseudopotential method implemented in the Vienna Ab Initio Simulation Package (VASP) code. [2][3] The Perdew-Burke-Ernzerhof (PBE) functional was employed to approximate the exchange-correlation (XC) energy for all DFT calculations. Total Coulomb-energy (EC) calculations were carried out using the supercell code. [4][5] DFT calculations were carried out using a Gamma-centered 2×2×1 k-point mesh for LixCoO2 and 1×1×1 k-point mesh for LixLa3AlyCozZr1.625Ta0.375O12. An energy cutoff of 600 eV as well as an energy and force convergence criterion of 10 -4 eV and 10 -2 eVÅ -1 , respectively, were used for all DFT calculations.
To find the atomic structure of the cubic Li7La3Zr2O12 (c-LLZO) with Al and Ta dopants, namely Li50La24Al1Zr13Ta3O96, we started with the c-LLZO structure (i.e., Li56La24Zr16O96) from our previous work. 6 Since computing EC values for all possible combinations of ( The formation energies of Al in different sites of charged and discharged LCO and the competing phases in the Li, Co, Al, O four elements system were obtained by performing first-principles calculation base on density functional theory (DFT). 7 The Projector Augmented-Wave (PAW) 2 pseudopotential implanted in Vienna Ab Initio Simulation Package (VASP) 3,8 was used for spinpolarized DFT calculation. The Perdew-Burke-Ernzerhof (PBE) 4 functional was adapted for the exchange correlation. DFT+U method was used for energy modification in the calculations of transition metal oxides. 9 The U value for Co, which is 3.32 eV, was taken from Materials Project. 10 To further obtain more accurate formation energy calculation, an energy correction (1.87 eV) for calculating Co containing oxides formation energies was applied. [11][12] Li27Co27O54 cell was built for discharged LCO, and Li15Co27O54 model was used for charged LCO. To obtain Al stability at Li layer octahedral site, Li layer tetrahedral site, and Co site in discharged LCO, Li24AlCo27O54

Calculation of Raman spectra of LCO by DFT
Raman spectra of LCO as well as LCO with Al substitution into the Li-and Co-site were computed using DFT-PBE calculation (Fig. 6a,b). The centers of  values computed by DFT-PBE for pure LCO are at 569 cm -1 and 436 cm -1 (Fig. 6a), which is 37 cm -1 and 44 cm -1 lower, respectively, compared to the experimental values (Fig. S5). Underestimation of vibrational frequencies is, however, typical for the DFT-PBE calculation and has also been reported for other systems in previous theoretical studies using the PBE functional. 13  for CoO6 octahedra in Al(Co)-LCO that are at the same layer as AlO6 octahedra are, however, larger (dCo−O = 1.930 Å and 1.931Å) and closer to those in the bare system leading to the appearance of two small peaks with 2 = 577 cm -1 and 572 cm -1 close to 2 = 569 cm -1 in the bare LCO.

Microstructural analysis
The microstructure was investigated by Scanning Electron Microscopy (SEM) with a Zeiss Ultra 55 microscope in back-scattering mode at 15 kV. Samples were embedded in epoxy and polished or used as fracture surface analysis. First SiC sandpaper up to #4000 was used followed by water free diamond suspensions (9 μm, 3 μm, and 1 μm). The surface electronic conduction was increased by sputtering of a thin Pt-(EM ACE200, Leica) or Au-layer (Cressington 108).

Electrochemical impedance analysis
In the Nyquist plots ( Fig. 2c) a semicircle in the high frequency range attributed to the total (bulk, b and grain boundary, gb) resistance of the solid electrolyte, a stretched semicircle in the mid-to low frequency range, and the Li-ion diffusion tail in the low frequency range is visible. The midto low frequency range is interpreted as a combination of the individual impedances of the anode and the cathode. The medium frequency contribution is assigned to the LCO/LLZO:Al,Ta impedance and the low frequency contribution to the LLZO:Al,Ta/In-Li impedance based on the observation made by Janek and coworkers, [14][15] Sakuda et al. 16 as well as in our previous analysis of the LCO/LLZO:Al,Ta system. 17 Often, also the capacitance values are used to assign the individual contributions. 15,[18][19] However, this approach requires the separation of the individual contributions within the EIS spectra. In our EIS spectra, only two capacitances are accessible. So, the contributions assigning based on the capacitance value is challenging (Table S1). Figure S1. Cross sectional SEM image of the prepared half-cell by FAST/SPS at 750 °C with 440 MPa applied pressure in Ar atmosphere. The LCO-LLZO:Al,Ta layer has a density of around 95% while the LLZO:Al,Ta separator layer has a density of around 92%.    Tables   Table S1. Fit results for ASSLB with a LCO-LLZO:Al,Ta cathode. R represents an ohmic resistor and CPE a constant phase element. The indices are: C: cable, B: bulk/grain, GB: grain boundary and Se/El: Solid electrolytes/electrode (cathode and anode). The exponent for the CPE is denoted as n. The capacity (C) was calculated with the following formula: C = (CPE · R) (1/n) / R.